MTJ Device Performance by Controlling Device Shape

ABSTRACT

A layered thin film device, such as a MTJ (Magnetic Tunnel Junction) device can be customized in shape by sequentially forming its successive layers over a symmetrically curved electrode. By initially shaping the electrode to have a concave or convex surface, the sequentially formed layers conform to that shape and acquire it and are subject to stresses that cause various crystal defects to migrate away from the axis of symmetry, leaving the region immediately surrounding the axis of symmetry relatively defect free. The resulting stack can then be patterned to leave only the region that is relatively defect free.

BACKGROUND 1. Technical Field

This disclosure relates generally to magnetic memory devices, specifically to the effect of the shape of a device on its performance.

2. Description of the Related Art

The crystalline property of the structure of a magnetic memory device, specifically that of the ferromagnetic/MgO tunneling barrier interface of a MTJ (magnetic tunneling junction) device, plays a very critical role in the device performance. In particular, the strain in the structure can significantly alter the properties of the device, for reasons to be discussed below.

Magnetic memory devices comprise a stack of layers in which two ferromagnetic layers, typically referred to as a reference layer and a free layer, are separated by a thin, non-magnetic dielectric layer, called a barrier layer. In the classical physics regime, a current of electrons going from one ferromagnetic layer to the other would be unable to pass through the barrier layer, which is non-conducting; but according to quantum mechanics, electrons can “tunnel” through the barrier layer if the right conditions exist in terms of the spin of the tunneling electrons and the magnetization directions of the two ferromagnetic layers on either side of the barrier. The conditions necessary for the electrons to successfully tunnel also depend on the quality of the interfaces between the barrier layer and the ferromagnetic layers. Imperfections in the interfaces make it difficult to achieve high TMR (Tunneling Magneto Resistance) values, which measure the ability of the electrons to successfully tunnel when the proper magnetization conditions are met. Such imperfections result from lattice mismatches between the ferromagnetic layers and the non-magnetic barrier layer and from defects that occur during crystal growth of materials. These undesirable qualities are associated with strain which, in turn, causes reduction in the TMR values, as has been documented, for example, in “Strain-enhanced tunneling magnetoresistance in MgO magnetic tunnel junctions,” by Li Ming Long et al., Scientific Reports 4, Article number 6505 (2014) and also “Tuning the magnetic anisotropy of CoFeB grown on flexible substrates” by Zhang Hao et al. in Chinese Physics B, Volume 24, Number 7-077501 (2015)

Given the fact that the shape of a crystalline layer affects the stresses and strains in the layer which, in turn, affects the growth of defects in that layer, controlling layer shape should permit a corresponding control of and even a purposeful manipulation of the stresses in an MTJ film stack. Therefore, such a shape controlling process should enable improvements in device performance, for example, improving the TMR and the coercivity of the device. More specifically, the new shape of a device, if properly designed and produced, can be expected to reduce the interface defect concentration and improve interface lattice epitaxy, all of which will improve device performance.

Attempts to influence device performance by shape control are known in the prior art, for example, Ahn et al. (U.S. Pat. No. 7,998,758) and Kim et al. (U.S. Pat. No. 9,305,928). However these attempts do not use the same methods or produce the same effects as will be described herein.

SUMMARY

A first object of the present disclosure is to provide a method of improving the performance of a layered MTJ device by controlling its shape.

A second object of the present disclosure is to provide such a method that reduces interfacial defect concentrations and lattice mismatches and improves lattice epitaxy, thereby creating a measurable improvement in TMR.

A third object of the present disclosure is to provide such a method that allows controlling and manipulating the shape of a layered device during its fabrication, in effect customizing its shape.

A fourth object of the present disclosure is to control the stresses in a TMJ layered film stack or similar device structure by controlling its shape so that the stresses thereby cause crystal defects to migrate towards regions of the structure that can be subsequently removed.

These objects will be achieved through the design and fabrication of a patterned, layered MTJ device during which its shape is controlled in such a manner that crystal defects such as vacancies, pinholes and dislocations within the stack are moved to an undesirable weak zone which can subsequently be removed. The relatively defect-free remaining portion of the device will have a higher coercivity and improved TMR values. The new device shape will, therefore, reduce the interface defect concentration and improve the interface lattice epitaxy, both results leading to improvement of device performance as, for example, measured by the improved TMR values.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a prior art MTJ device.

FIG. 2 schematically shows a dome-shaped bottom electrode for the device in FIG. 1 in accord with the method of the present disclosure.

FIG. 3 schematically shows the effects on the formation of a layered MTJ stack when the stack is formed over the dome-shaped electrode of FIG. 2.

FIG. 4 is a schematic illustration showing the effects of patterning the stack of FIG. 3, so that laterally disposed regions of the stack containing defects resulting from its formation over a dome-shaped electrode are eliminated.

FIG. 5 is a schematic illustration of an alternative electrode which is concave rather than convex as is the dome-shaped electrode.

FIG. 6 is a schematic illustration, analogous to that of FIG. 3, but now showing the effects of forming a MTJ stack over the concave electrode of FIG. 5.

FIG. 7 is a schematic illustration showing the effects of patterning the stack of FIG. 6, so that laterally disposed regions of the stack containing defects resulting from its formation over a concave electrode are eliminated.

DETAILED DESCRIPTION

Referring first to FIG. 1, there is shown a standard patterned MTJ (Magnetic Tunneling Junction) layered device such as is commonly used in a random access memory (RAM) array. Although a specific substrate is not necessary to define the device, for specificity we will consider the device to be formed on a functional substrate, such as a CMOS substrate, that may already include circuitry and conducting vias to access various elements of the devices in a RAM array.

The device is formed by first providing a CMOS substrate 10 on which to form it. A blanket bottom electrode (BE) 20 is deposited on the CMOS substrate so that it properly contacts the vias in that substrate. There is then deposited on top of the bottom electrode 20, in succession, a series of horizontal layers that will comprise the MTJ stack. Those layers include, a seed layer 30, a pinned magnetic layer 40, a tunnel barrier layer 50, a free magnetic layer 60 and capping layer 70. A hard mask 80 is deposited on the capping layer. The hard mask is then patterned by a standard photolithographic process (not shown) and the film stack is then patterned using the patterned hard mask as a guide. Note that the bottom electrode is also shown as patterned and it is surrounded laterally by dielectric fill material 90.

The process steps leading to the formation of the typical prior art MTJ as shown in FIG. 1 and described in detail above, can be summarized in the flow diagram of Table 1, below.

We will now describe the method of the present disclosure as illustrated with reference to FIG. 2-FIG. 4. We note at the outset that the method will be described with reference to an MTJ device, but it is applicable to any layered, crystalline structure that can be expected to accumulate various types of defects as it is grown.

Like the prior-art method described by the process flow chart in FIG. 1, above, the method of the present disclosure also begins with a CMOS substrate 10 (although other substrates are also possible) and, on this substrate a bottom electrode layer 20 formed of conducting material is deposited. However, the present method deviates from the prior-art method above by patterning and etching the electrode to form a “dome” (i.e., a symmetrical convex) shape 25, as is shown in FIG. 2. Note that the “dome” or convex shape, is here axially (about a horizontal axis) symmetric, so it is more correctly a cylinder (or a semi-circular cylinder if the convex shape has a semi-circular cross-section).

There are several methods by which the electrode can be formed with a surface having a convex, cylindrical shape, or indeed, by which an electrode can be formed with various surface shapes other than a convex shape, such as a concave shape to be discussed below. For example, an initial etch can be performed on a layer of electrode material to create a solid rectangular prism and then a partial etch (or series of partial etches) can be performed that successively removes corners of the electrode to give it the rounded surface shape. Alternatively, a sputter etch can be performed to produce a rounded surface shape. To form a concave surface, isotropic physical etching can be performed.

Referring next to schematic FIG. 3, there is shown the result of a sequential deposition process in which there is first deposited on the convex cylindrical electrode 25, a seed layer 35, which is followed by a pinned layer 45, a barrier layer 55, a free layer 65, a capping layer 75 and a hard mask layer 85. These layers will all conform to the shape of the electrode, by each layer conforming, in sequence, to the shaped layers beneath it. For definiteness we note that the above process steps can be applied to a typical MTJ structure having a seed layer of Ta, Ru, W or NiCr with a thickness range of between approximately 50-500 A (angstroms), ferromagnetic layers for pinned and free layers formed of CoFeB with a thickness range between approximately 10-100 A, a tunneling barrier layer of AlOx or MgO of thickness between approximately 5-50 A and a capping layer of Ta, W or Mg with a thickness range between approximately 10-100 A. The convex electrode can have a thickness of between approximately 100-1000 nm and have a width between approximately 50-500 nm.

Due to the underlying convex shape, as successive layers are formed and develop their crystalline structure, the inevitable defects produced by the crystal growth mechanism, such as pinholes, vacancies and dislocations, will move away (migrate) from the top of the convexity of each successively formed layer and accumulate at the corners where the underlying curved shape of the electrode becomes a horizontal layer. This is what is called the “weak” region of the structure (see the region surrounded by an ellipse 90).

The region of the stack containing a small region to either side of the peak of the convex structure will be relatively defect free. By controlling the slope (curvature) of the convexity, the film strain can be manipulated, and the lattice mismatch and interfacial defect concentration can be reduced within a desired region symmetrically disposed to either side of the peak of the convex structure. Note that the width of the convexity that can be considered relatively defect free can cover most of the total width of the curved region with exception of the weak regions 90 at the corners. In most cases, more than half of the width of the convexity, centered about its highest point can be considered defect free. For example, an electrode having a base width of 200 nm will produce a defect free device shape of at least 100 nm.

After the convex structure is patterned to produce the vertical stack, which is actually a cylindrical slice, as shown in schematic FIG. 4, the gaps to either side 110 are filled with a gap-filling dielectric material (not shown) and the upper surfaces of the stack and its surrounding dielectric are planarized and polished to remove the mask layer in preparation for further process integration. Note that Table 2 below is a process flow chart that briefly describes the sequence of steps that lead to the structure in FIG. 4 and are discussed above.

Referring next to schematic FIG. 5, there is shown an alternative embodiment of the present process where the bottom electrode 27 is shaped to have a concave upper surface and is otherwise axially symmetric.

Referring next to FIG. 6, there is shown that the film stack of the MTJ (or any layered crystalline construction) can be deposited over the concave bottom electrode 27 in the same manner as it was deposited over the previously described convex electrode. FIG. 6, shows schematically the result of a sequential deposition process in which there is first deposited on the concave electrode 27 a seed layer 37, which is followed by a pinned layer 47, a barrier layer 57, a free layer 67, a capping layer 77 and a hard mask layer 87. These layers will all try to conform to the concave shape of the electrode, by each layer conforming, in sequence, to the concave-shaped layers beneath it. Note that the material layers can be formed of the same materials and dimensions as described above for the convex structure.

Due to the underlying concave shape, as successive layers are formed and develop their crystalline structure, the inevitable defects produced by the crystal growth mechanism, such as pinholes, vacancies and dislocations, will move away (migrate) from the bottom point of the concavity of each layer and accumulate at the upper corners where the concavity, discontinuously, merges with the horizontal layer of the substrate 10. The region of the stack containing a small region to either side of the concave minimum will be relatively defect free. This region surrounding the discontinuity, called a “weak” region, is shown enclosed in an elliptical region 100. It is at this weak region that defects will accumulate. By controlling the shape of the electrode 27 the film strain in the sequentially deposited layers can be manipulated, and the lattice mismatch and interfacial defect concentration can be reduced in a specific region.

After the concave-shaped structure is patterned to produce the vertical stack that now contains the defect-free bottom of the concavity, as shown in schematic FIG. 7, the gaps to either side 110 are filled with a gap-filling dielectric material and the upper surfaces of the stack and its surrounding dielectric are planarized and polished in preparation for further process integration (not shown).

Note that Table 3 below is a process flow chart that briefly describes the sequence of steps that lead to FIG. 7 and are discussed above.

As is finally understood by a person skilled in the art, the detailed description given above is illustrative of the present disclosure rather than limiting of the present disclosure. Revisions and modifications may be made to methods, materials, structures and dimensions employed in forming and providing a layered crystalline magnetic device such as an MTJ device, whose layer strain is controlled during layer formation to reduce crystal defects and thereby to improve device performance, while still forming and providing such a structure and its method of formation in accord with the spirit and scope of the present invention as defined by the appended claims. 

What is claimed is:
 1. A method of controlling effects of layer strain while forming a magnetic thin film device, comprising: providing a substrate having a flat upper surface; forming on the substrate an electrode layer having a flat upper surface; symmetrically shaping said flat upper surface of said electrode layer to form a curved surface having an axis of symmetry; sequentially depositing a layered device structure over the symmetrically shaped upper surface of said electrode, whereby each layer in said layered device structure acquires a shape corresponding to said symmetrically shaped upper surface of said electrode with laterally extending portions over the flat upper surface of said substrate; then using a hard mask layer as a guide, removing symmetric portions of said layered device structure that are laterally disposed to each side of said axis of symmetry, leaving thereby a remaining portion of said layered device structure, wherein said removed portions contain weak regions where said curved surfaces meet said surrounding flat surfaces; wherein layers in said removed portions contain vacancies, crystal defects, pinholes and dislocations that have migrated to said weak regions and collected therein during formation of said layered device structure and, wherein a remaining portion of said layered device structure is relatively free of vacancies, crystal defects, pinholes and dislocations.
 2. The method of claim 1 further including surrounding said remaining portion of said layered device structure by a dielectric fill layer and planarizing said dielectric surrounded remaining portion in preparation for further process integration.
 3. The method of claim 1 wherein said vacancies, crystal defects, pinholes and dislocations migrate to said laterally disposed regions as a result of strains induced in layers due to their curvature.
 4. The method of claim 1 wherein said curved region is convex.
 5. The method of claim 1 wherein said curved region is concave.
 6. The method of claim 1 wherein said layered device structure comprises: a bottom electrode having a surface that is symmetrically curved about an axis; upon which is sequentially formed: a seed layer; a ferromagnetic pinning layer; a tunneling junction layer; a ferromagnetic free layer; a capping layer; and a hard mask layer.
 7. The method of claim 6 wherein said seed layer is a layer of Ta, Ru, W or NiCr with a thickness range of between approximately 50-500 A (angstroms); said ferromagnetic free layers and pinning layers are formed of CoFeB with a thickness range between approximately 10-100 A; said tunneling barrier layer is formed of AlOx or MgO of thickness between approximately 5-50 A and said capping layer is formed of Ta, W or Mg with a thickness range between approximately 10-100 A.
 8. The method of claim 6 wherein said electrode has a thickness of between approximately 100-1000 nm and has a width between approximately 50-500 nm.
 9. The method of claim 8 wherein two regions, symmetrically disposed about the axis of symmetry of said layered device structure, are removed; whereby a remaining portion of said layered device structure has a width between approximately 50-500 nm.
 10. A shape-controlled magnetic layered device having strain-free and defect-free crystalline layers, comprising: A bottom electrode, having an initially axially symmetric curved surface; a sequence of layers formed on said curved electrode; wherein said sequence includes layers having a crystalline structure and wherein strain in said layers is controlled by said layers conforming to said curved bottom electrode surface during formation; wherein each of said sequence of layers is defect-free within a first region symmetrically disposed about the axis of symmetry of said electrode, but wherein defects have accumulated within a second region laterally disposed beyond said first region; wherein said second region has been removed and said remaining first region is defect free.
 11. The device of claim 10 wherein a dielectric fill layer is deposited to either side of said first region and the upper surfaces are planarized and polished and prepared for further integration.
 12. The device of claim 10 wherein said initial surface shape of said bottom electrode is convex.
 13. The device of claim 10 wherein said initial surface shape of said bottom electrode is concave.
 14. The device of claim 11 wherein said layers comprise a TMJ device; comprising: a bottom electrode; a seed layer; a ferromagnetic pinned layer; a tunneling barrier layer; a ferromagnetic free layer; and a capping layer.
 15. The device of claim 14 wherein said bottom electrode is a layer of conducting material, said seed layer is a layer of Ta, Ru, W or NiCr with a thickness range of between approximately 50-500 A (angstroms); said ferromagnetic free layers and pinning layers are formed of CoFeB with a thickness range between approximately 10-100 A; said tunneling barrier layer is formed of AlOx or MgO of thickness between approximately 5-50 A and said capping layer is formed of Ta, W or Mg with a thickness range between approximately 10-100 A.
 16. The device of claim 15 wherein said bottom electrode is initially formed with a convex axially symmetric surface shape.
 17. The device of claim 15 wherein said bottom electrode is initially formed with a concave axially symmetric surface shape. 